Adaptive thermogenesis was detected in 60% of participants in the early stages of calorie restriction. It became significant on the third day.

It was partly responsible for the decrease in energy expenditure, accounting for 108 calories/day.

Of these 108 calories per day, 36 were explained by changes in the composition of fat-free mass, leaving 72 calories for “true” adaptive thermogenesis.

Factors that were associated with adaptive thermogenesis included reduced insulin levels, heart rate, kidney function and body fluid balance. Increased formation of glucose in the liver also appeared to play a role.

It was also not linked to hormonal changes, such as in leptin, ghrelin, adiponectin and thyroid hormone levels (T3).

Lastly, adaptive thermogenesis appears to have no long-term effects on weight or fat regain after short-term calorie restriction. The effect was reversed within two weeks of re-feeding.

Here you can see how energy expenditure changes from adaptive thermogenesis developed during the course of the study:

As you can see, the increase during the re-feeding phase completely reversed the decrease seen during the calorie restriction phase.

Several other parameters changed significantly during 3 weeks of severe calorie restriction:

Fat mass: Decreased by 114 grams per day.

Lean mass: Decreased by 159 grams per day (muscles and organs combined, see below).

Muscle mass: Decreased by 5%.

Liver mass: Decreased by 13%.

Kidney mass: Decreased by 8%.

Heart rate: Went down by 14%.

Blood pressure: Went down by 7%.

Creatinine clearance: Went down by 12%.

Energy cost of walking: Decreased by 22%.

Sympathetic nervous system activity: Decreased by 38%.

Leptin levels: Decreased by 44%.

Insulin levels: Decreased by 54%.

Adiponectin levels: Decreased by 49%.

Triiodothyronine (T3): Decreased by 39%.

Testosterone: Decreased by 11%.

Main Conclusions

The main conclusions of the study were as follows:

Adaptive thermogenesis is modest and kicks in during the early stages of starvation, shortly after calorie restriction starts. It is maintained as long as you keep restricting calories.

Adaptive thermogenesis appears to have no long-term effects on weight or fat regain after short-term calorie restriction.

Adaptive thermogenesis was partially explained by reductions in heart rate and kidney function, and increased formation of glucose in the liver.

In early calorie restriction, adaptive thermogenesis is also associated with a drop in insulin levels.

Limitations

The study has no apparent limitations. It was tightly controlled and had highly significant results.

However, since the participants were limited to healthy, sedentary, non-obese men, the results may not be generalized to all people.

For example, it is possible that the dynamics of of adaptive thermogenesis are vastly different in people with obesity.

What Do Other Studies Say?

In general, adaptive thermogenesis (AT) is considered to be an automatic response to food shortage.

In the Minnesota Starvation Experiment from 1950, resting energy expenditure declined by 39% (600 calories/day). About 35% of this decline was not caused by changes in body composition, which indicates that AT also played a role (4).